spec/backend_client_spec: add missing 'it' clauses
spec/backend_client_spec: add missing 'it' clauses

The tests failed some times because of those missing 'it' clauses.

This was the error:
```
attempt to index upvalue 'test_backend'
```

3scale/apicast,3scale/docker-gateway,3scale/apicast,3scale/apicast,3scale/apicast,3scale/docker-gateway